fundamental to field crop production (Osaki 1993, Osaki et al. 1992, 1993a, b, c, d, e,
1996a, b). As carbon is assimilated by leaves and nitrogen is absorbed from roots, it
is hypothesized that root–shoot interactions are essential to plant growth and high
yields (Osaki et al. 1995e, 1996a, b, 1997). In superhigh yield trials, it was found that
high root activity, especially at the ripening stage after flowering, is crucial in
achieving superhigh yields of several field crops (Osaki et al. 1991a, b, 1993a, e;
Osaki 1995a, b; Samejima et al. 2004, 2005; He et al. 2005). To maintain high root
activity, the carbohydrates (sucrose or organic acids) assimilated in leaves must be
supplied to the roots.
1.8.2.2 P Metabolism (Fig. 1.20)
Phosphorus (P) is a key element regulating carbon metabolism in leaves (Nanamori
et al. 2004; Shinano et al. 2005; Begum et al. 2005, 2006; Wasaki et al. 2006).
• 1st important process: The high-energy P compounds ATP and NADPH are
produced through PSII and PSI.
• 2nd important process: Inorganic phosphorus (Pi) must be transported with
triose-P from the chloroplast to the cytosol as regulated by TPT (transporter) or
with PEP from the cytosol to the chloroplast as regulated by PPT (transporter).
These processes indicate that Pi regulates photosynthate transportation, which
also indirectly regulates the photosynthetic rate.
• 3rd important process: The PEP transported from the cytosol to the chloroplast is
metabolized into the precursors of high molecular weight compounds: lignin,
aromatic amino acids, fatty acids, and pigments. It is assumed that these high
molecular weight compounds require a high amount of energy for their construction, and that high levels of these compounds would cause low plant productivity.
However, as the precursors of high molecular weight compounds are composed
in the energy-rich chloroplasts, energy loss is not a serious threat to productivity.
• 4th important process: Under P deficiency or P-limited conditions, PEPC and
PPT are activated, which contributes to more Pi cycling processes, such as Pi pool
formation.
In conclusion, under P deficiency or P-limited conditions, the TCA cycle and
precursors of high molecular weight compound pathways are activated (Shinano
et al. 2001). The TCA cycle is normally cyclic to produce energy; however, the TCA
cycle is also a key metabolic pathway for producing organic acids. These organic
acid metabolism pathways are combined with nitrogen metabolism (e.g.,
GS/GOGAT catabolization), and then amino acids are produced. Leguminosae
plant metabolisms are oriented to the TCA-amino acid pathway, which is controlled
by the PEPC-SPS regulation balance, and they show higher PEPC activity than
Gramineae (Osaki et al. 1995d, Nakamura et al., 1997, Okazaki et al., 2005).
1 Basic Information About Tropical Peatland Ecosystems
33
1996a, b). As carbon is assimilated by leaves and nitrogen is absorbed from roots, it
is hypothesized that root–shoot interactions are essential to plant growth and high
yields (Osaki et al. 1995e, 1996a, b, 1997). In superhigh yield trials, it was found that
high root activity, especially at the ripening stage after flowering, is crucial in
achieving superhigh yields of several field crops (Osaki et al. 1991a, b, 1993a, e;
Osaki 1995a, b; Samejima et al. 2004, 2005; He et al. 2005). To maintain high root
activity, the carbohydrates (sucrose or organic acids) assimilated in leaves must be
supplied to the roots.
1.8.2.2 P Metabolism (Fig. 1.20)
Phosphorus (P) is a key element regulating carbon metabolism in leaves (Nanamori
et al. 2004; Shinano et al. 2005; Begum et al. 2005, 2006; Wasaki et al. 2006).
• 1st important process: The high-energy P compounds ATP and NADPH are
produced through PSII and PSI.
• 2nd important process: Inorganic phosphorus (Pi) must be transported with
triose-P from the chloroplast to the cytosol as regulated by TPT (transporter) or
with PEP from the cytosol to the chloroplast as regulated by PPT (transporter).
These processes indicate that Pi regulates photosynthate transportation, which
also indirectly regulates the photosynthetic rate.
• 3rd important process: The PEP transported from the cytosol to the chloroplast is
metabolized into the precursors of high molecular weight compounds: lignin,
aromatic amino acids, fatty acids, and pigments. It is assumed that these high
molecular weight compounds require a high amount of energy for their construction, and that high levels of these compounds would cause low plant productivity.
However, as the precursors of high molecular weight compounds are composed
in the energy-rich chloroplasts, energy loss is not a serious threat to productivity.
• 4th important process: Under P deficiency or P-limited conditions, PEPC and
PPT are activated, which contributes to more Pi cycling processes, such as Pi pool
formation.
In conclusion, under P deficiency or P-limited conditions, the TCA cycle and
precursors of high molecular weight compound pathways are activated (Shinano
et al. 2001). The TCA cycle is normally cyclic to produce energy; however, the TCA
cycle is also a key metabolic pathway for producing organic acids. These organic
acid metabolism pathways are combined with nitrogen metabolism (e.g.,
GS/GOGAT catabolization), and then amino acids are produced. Leguminosae
plant metabolisms are oriented to the TCA-amino acid pathway, which is controlled
by the PEPC-SPS regulation balance, and they show higher PEPC activity than
Gramineae (Osaki et al. 1995d, Nakamura et al., 1997, Okazaki et al., 2005).
1 Basic Information About Tropical Peatland Ecosystems
33